A Spindt-type electrode, a carbon nanotube (CNT)-type electrode and the like have been known as conventional cold cathode-type electron emitting elements, which have been studied on applications to the field of FED (Field Emission Display). The elements are operated in such a manner that a voltage is applied to a pointed end to form a strong electric field of about 1 GV/m and to emit electrons with the help of a tunneling effect.
There has been heretofore present an idea that such an electron emitting element is operated in the atmosphere and applied to a charger or an electrostatic latent image forming device. For example, there has been proposed a method in which a Spindt-type cold cathode is operated in the atmosphere to emit electrons into the atmosphere, to ionize gas molecules into ions as charged particles and to form an electrostatic latent image (see Japanese Laid-Open Patent Publication No. 06-255168). Besides, the result of a research on a carbon nanotube operated in the atmosphere has been reported (see Yamaguchi and three others, “Development of High Efficiency Electron Source for Image Recording with Carbon Nanotube,” Japan Hardcopy 97 articles, The Imaging Society of Japan, July 1997, pp 221-224). As seen from the documents, suggestion has been given on a possibility of applying an electron emission element as an electron source for an electrophotograph charger or electrostatic latent image forming device.
The two types of electron emitting elements, however, have a strong electric field in the vicinity of a surface of an electron emitting section as described above, which makes it easy that emitted electrons acquire energy larger than the electric field to ionize gas molecules. This has resulted in a problem that plus ions generated by ionization of gas molecules are accelerated by the strong electric field in the direction toward the element surface and collide with the element surface, causing element breakdown due to sputtering.
There have been known other type cold cathodes such as an MIM (Metal Insulator Metal) type and an MIS (Metal Insulator Semiconductor) type. Those are a surface emission-type electron emitting element working in a way such that electrons are accelerated using a quantum size effect and a strong electric field and caused to be emitted from a flat element surface. The electron emitting elements have no necessity for a strong electric field outside of the element since electrons are accelerated inside of the element and emitted. Hence, an electron emitting element of the MIM type or the MIS type can solve a problem that the element is broken down by sputtering through ionization of gas molecules, which occurs in the electron emitting element of the Spindt type or the CNT type.
An electron emitting element in which electrons injected into a porous semiconductor are accelerated in an electric field, forced to pass through a surface metal thin film with the help of a tunneling effect and finally emitted into a vacuum has been proposed as an electron emitting element belonging to the MIS type using a quantum size effect of a porous semiconductor (for example, porous silicon) formed by an anodic oxidation treatment on a semiconductor (see Japanese Laid-Open Patent Publication No. 08-250766). A cold cathode made of such a porous semiconductor has a great merit that an element can be fabricated by means of an extremely simple, convenient, low-cost method adopting anodic oxidation.
In a case where such an element is operated in the atmosphere, however, a problem has newly occurred that various gas molecules are adsorbed on a surface of the element to change an electric characteristic or the like of the semiconductor and to thereby reduce an electron emission current.
The surface of a cold cathode of the MIM type or the MIS type inside which element electrons are accelerated is constituted generally of a metal thin film playing a role as an upper electrode applying an electric field to the inside of the element. Since electrons accelerated in the inside of the element, however, are emitted into a vacuum tunneling through the surface metal thin film, an tunneling effect enhanced with a smaller film thickness increases an electron emission quantity. A thickness of the metal film by which the two roles are established simultaneously has been appropriate in the range of from several nm to tens of nm. For example, in Japanese Laid-Open Patent Publication No. 08-250766, there is disclosed an example with a thickness of a metal thin film of 15 nm.
Cold cathodes of the MIM type and the MIS type have difficulty forming a dense metal film because of a very thin film on the surfaces thereof and almost no barrier effect to gas molecules is exerted. Therefore, in a case where an electron emitting element is operated in the atmosphere, a problem arises that gas molecules intrude into an inside semiconductor layer to change an electric characteristic or the like of the semiconductor to thereby reduce an electron emission current.